Why is the ring of vertices not complete?
I think I know what you did. You had a vert or an edge of the line selected and then added the stuff you wanted to hide with box select. The previous selected vert or edge was also still selected.

You could have hit A to deselect it before the box select.

What you just need to do now is press Alt-H to unhide everything (or Mesh -> Show/Hide -> Show hidden), do your selection again and hide it again.